I suggest you ...

indicate subtask number and status next to parent task title

I imagine something similar to the overlapping colored number bubbles that show number of tasks in a category on iOS, but have this for composite tasks on the desktop version.

In other words, I want to see how many subtasks are in a composite even when it is closed in tree view. More importantly, I want to know when a composite task contains inactive tasks that are hidden without having to show them. It is very valuable to be able to quickly know whether inactive tasks are present.

Colored text could work, with # in GREY to indicate the presence of inactive subtasks; # in green to indicate complete subtasks; maybe also a # for total subtasks and maybe for the other states too (late, due soon, overdue)

(side note: this same feature might be nice as an option for categories, thus making the desktop categories have the same indications as are already on iOS, but the main request is for composite tasks showing this.)

Also, here's a separate thought on how this could be implemented. I think this is NOT as good as the idea in the main request (and would not solve the issue for that other request), but it would be something:
Add a section to the bottom status bar that indicates particular information about whatever is selected. When a composite task is selected, the status bar could indicate the statistics about how many subtasks the selected task contains and what what their status is (shown/hidden, ideally inactive indicated separately from completed)